Method for producing acetone cyanhydrin and the subsequent products thereof by specific cooling

ABSTRACT

The present invention relates in general terms to a process for preparing acetone cyanohydrin, comprising as steps:
         A. contacting acetone and hydrocyanic acid in a reactor to give a reaction mixture, the reaction mixture being circulated, to obtain acetone cyanohydrin;   B. cooling at least some of the reaction mixture by flowing it through a cooling region of a cooler, the cooler including one cooling element or at least two cooling elements;   C. discharging at least a portion of the acetone cyanohydrin obtained from the reactor,
 
the volume of the cooling region of the cooler based on the total internal volume of the cooler being greater than the volume of the cooling element or of the at least two cooling elements of the cooler, to a process for preparing an alkyl methacrylate, to a process for preparing a methacrylic acid, to an apparatus for preparing alkyl methacrylates, to a process for preparing polymers based at least partly on alkyl methacrylates, to the use of the alkyl methacrylates obtainable from the process according to the invention in chemical products, and to chemical products based on the alkyl methacrylates obtainable by the processes according to the invention.

The present invention relates in general terms to a process forpreparing acetone cyanohydrin, to a process for preparing an alkylmethacrylate, to a process for preparing methacrylic acid, to anapparatus for preparing alkyl methacrylates, to a process for preparingpolymers based at least partly on alkyl methacrylates, to the use of thealkyl methacrylates obtainable from the process according to theinvention in chemical products, and to chemical products based on thealkyl methacrylates obtainable by the processes according to theinvention.

Acetone cyanohydrin is one of the important starting components in thepreparation of methacrylic acid and alkyl methacrylates. These twocompounds are monomers which are of great significance for a number ofbulk plastics and whose demand is rising constantly as a result of theincreasing rise in the use of these bulk plastics, such that the acetonecyanohydrin starting material is also required in increasingly greateramounts. Acetone cyanohydrin is obtained, for example, from hydrocyanicacid and acetone or else from acetone and hydrocyanic salts. Incomparison to acetone, hydrocyanic acid and hydrocyanic salts aresignificantly more toxic compounds which have to be handled with verygreat care and hence with a high level of technical safety measures. Inorder to keep these safety measures within commercially acceptablelimits, the conduct of the reaction in the preparation of acetonecyanohydrin is of particular significance. In addition to the generalinterest in obtaining high yields with minimum reactant concentrations,an additional factor here is the safety interest that, with the acetonecyanohydrin as the product of this reaction, a minimum amount ofhydrocyanic acid as the reactant of this reaction be present in theworkup of acetone cyanohydrin for further processing.

In the prior art, EP 1 371 632 A1, for example, addresses this safetyaspect by a process for preparing acetone cyanohydrin in which a metalcyanide composition is used. This procedure is disadvantageousespecially as a result of the metal cyanide intermediate when a plantfor preparing hydrocyanic acid and a plant for preparing acetonecyanohydrin are present on the same site, and the hydrocyanic acidprepared can thus be fed directly into the plant for preparing acetonecyanohydrin.

In the present context, there was therefore a general desire to at leastpartly or even completely eliminate the disadvantages arising from theprior art.

It was a further object of the present invention, by virtue of a maximumconversion, to keep the safety measures which have to be observed inparticular in the purification of acetone cyanohydrin as low aspossible.

It was a further object of the invention to provide a process forpreparing acetone cyanohydrin in which a minimum level of by-products isformed.

In addition, it was an object of the invention to improve thepreparation of methacrylic acid or alkyl methacrylates overall, in orderthus to obtain a less expensive route to polymers based thereon.

A contribution to the solution of at least one of the abovementionedobjects is made by the subject matter of the category-forming claims,the subclaims dependent thereon constituting preferred embodiments ofthe present invention.

The present invention thus relates to a process for preparing acetonecyanohydrin, comprising as steps:

-   -   A. contacting acetone and hydrocyanic acid in a reactor to give        a reaction mixture, the reaction mixture being circulated, to        obtain acetone cyanohydrin;    -   B. cooling at least some, preferably at least 10% by weight,        preferentially 50% by weight and more preferably at least 70% by        weight, based in each case on the reaction mixture, of the        reaction mixture by flowing it through a cooling region of a        cooler, the cooler including one cooling element or at least two        cooling elements;    -   C. discharging at least a portion of the acetone cyanohydrin        obtained from the reactor,        the volume of the cooling region of the cooler based on the        total internal volume of the cooler being greater than the        volume of the cooling element or of the at least two cooling        elements of the cooler.

The reaction mixture can be circulated preferably in a so-called loopreactor, in which the reaction mixture is pumped in circulation,preferably through a pipe system. The flow of the reaction mixturethrough the cooling region can be effected firstly by virtue of thecooling region directly forming part of the loop reactor. In anotherembodiment, it is, however, possible that, as well as the circuit of theloop reactor, a cooling region is provided, in which some of thereaction mixture which circulates within the loop reactor is passed inparallel to the loop reactor. In general, the reaction mixturecirculates with a volume flow rate in a range of about 30 to about 700m³/h, preferably in a range of about 100 to about 500 m³/h and morepreferably in a range of about 150 to about 400 m³/h. The flow of thereaction mixture through the cooling region may on the one hand bewithin the same aforementioned rate ranges. Moreover, it may bepreferred for the cooling region to be flowed through within a range ofabout 10 to 50 times, preferably in a range of about 20 to 40 times andmore preferably in a range of about 25 to 35 times by a particularamount of the reaction mixture. In the cooling region, the reactionmixture is brought to a temperature in a range of about 0 to about 50°C., preferably in a range of about 20 to 45° C. and more preferably in arange of about 30 to 40° C. It is desired to keep the reaction mixturesubstantially isothermal. It is also preferred that, at least in thecooling region, a pressure in a range of about 0.1 to about 10 bar,preferably in a range of about 0.5 to about 5 bar and more preferably ina range of about 0.9 to 1.5 bar is present. It is also preferred that,at least in the cooling region, a pH in a range of about 5 to about 9,preferably in a range of 6 to about 8 and more preferably in a range ofabout 6.5 to about 7.5 is present.

In general, the cooling region may have any configuration which is knownto those skilled in the art and appears to be suitable. For instance,the cooling region of a cooler often has one or at least two coolingelements which ensure the transfer of heat between the medium which iscooling, the reaction mixture here, and the cooling medium present inthe cooling element(s). The total internal volume of the coolergenerally refers to the volume specified by the manufacturer of thecooler. Often, this volume is the volume of the region in which the heattransfer takes place. Usually, tubes leading to the cooler and leadingaway from the cooler are not part of the total internal volume of thecooler, since they have no cooling function. The volume of the coolingregion is normally determined by subtracting the volume of the coolingelement provided in the cooling region or of the cooling elementsprovided there from the total internal volume of the cooler. The volumeof one cooling element is generally greater than the coolant volumecapacity of this cooling element, since the components of the coolingelement are also taken into account in the determination of its volume.It may be preferred in accordance with the invention that the volume ofthe cooling region is at least about 1.01 times, preferably at leastabout 1.1 times and more preferably at least about 1.5 times as great asthe volume of the cooling element or of the cooling elements of thecooler.

It is also preferred in the process according to the invention that atleast some reaction mixture flows in a cooler flow direction differentfrom the main flow direction at least during the cooling. The main flowdirection is determined as the axis between the entry point of thereaction mixture into the cooler and the exit point of the reactionmixture from the cooler. According to the invention, it may be preferredfor at least about 5%, preferably at least about 20% and preferentiallyat least about 40% of the reaction mixture to flow in a cooler flowdirection other than the main flow direction within the cooler duringthe cooling. It is also possible in accordance with the invention thatat least one part-stream, preferably at least 10% by volume,preferentially at least 20% by volume and more preferably at least 50%by volume of the reaction mixture fed into the cooling region isconducted past an outer wall which limits the cooling region. Thisconduct past an outer wall is preferably effected by virtue ofcirculation streams which preferably extend in spiral form from theentry of the reaction mixture into the cooling region to the exit of thereaction mixture from the cooling region. These flow conditions mayeither be visualized in a real situation, for example by adding a dyesuch as fuchsin to threads, or simulated by means of suitable computerprograms and mathematical models.

It is also preferred in accordance with the invention that the coolerflow direction is obtained by deflecting the reaction mixture. In turn,it is preferred here that the reaction mixture is deflected by one or atleast two deflecting means provided in the cooler or connected to thecooler. In general, these deflecting means may be provided in allregions of the cooler or be connected to the cooler. It may also bepreferred that the deflecting means are arranged principally in theregion of the cooler in which the reaction mixture enters the coolerwith comparatively high flow rate. Thereafter, the deflecting meansdensity decreases starting from the entry region of the reaction mixtureto the exit region of the reaction mixture from the cooler.

In principle, useful deflecting means are all devices which are known tothose skilled in the art and appear to be suitable. Deflecting meansparticularly preferred in accordance with the invention are injectingelements, stirring elements or baffle elements, or a combination of atleast two thereof.

Useful injecting elements include all devices which are known to thoseskilled in the art and inject the reaction mixture. These may distributethe reaction mixture within the cooling region firstly by virtue of thepressure of the reaction mixture as passive jets and secondly by virtueof active jets or similar devices. Useful stirring means are likewiseall stirrers which are known to those skilled in the art and appear tobe suitable, such as paddle stirrers or screw stirrers. Useful baffleelements are in principle also all suitable devices known to thoseskilled in the art. These are in particular flat internals such asplates, which are known, for example, from static mixers for mixing anddistribution. It is preferred that the deflecting means is provided inthe cooling region. Thus, the baffle elements in particular may bemounted either on the walls of the cooling region or on the walls of thecooling elements or on both.

Useful cooling elements are all of those which are known in generalterms to those skilled in the art and appear to be suitable. It ispossible here that the cooling elements are elongated hollow bodies withcoolant flow. The cooling elements may have a rod-shaped or plate-shapedconfiguration. Thus, tube bundle or plate coolers, or a combination ofthe two, are useful, particular preference being given to tube bundlecoolers. In tube bundle coolers, it is also preferred that thedeflecting means, preferably designed as baffle elements, are present onthe outer wall of the cooler, pointing into the cooling region.

It is also preferred in the process according to the invention that theresidence time of the reaction mixture in the cooler is selected suchthat a minimum level of by-products are formed at maximum conversions toacetone cyanohydrin. For instance, the residence time of the reactionmixture in the cooler is in a range of about 0.1 to about 2 h,preferably in a range of about 0.2 to about 1.5 h and more preferably ina range of about 0.3 to about 1 h.

The present invention further relates to a process for preparing analkyl methacrylate, comprising as steps:

-   a. preparing an acetone cyanohydrin by the process according to the    invention;-   b. contacting the acetone cyanohydrin with an inorganic acid to    obtain a methacrylamide;-   c. contacting the methacrylamide with an alcohol to obtain an alkyl    methacrylate;-   d. optionally purifying the alkyl methacrylate.

In addition, the present invention relates to a process for preparingmethacrylic acid, comprising as steps:

-   α) preparing an acetone cyanohydrin by a process according to the    invention;-   β) contacting the acetone cyanohydrin with an inorganic acid to    obtain a methacrylamide;-   γ) reacting the methacrylamide with water to give methacrylic acid.

The present invention further relates to an apparatus for preparingalkyl methacrylates, comprising, connected to one another influid-conducting form:

-   -   a plant element for preparing acetone cyanohydrin, followed by;    -   a plant element for preparing methacrylamide, followed by;    -   a plant element for preparing alkyl methacrylate, optionally        followed by;    -   a plant element for purifying the alkyl methacrylate, optionally        followed by;    -   a plant element for polymerization, optionally followed by;    -   a plant part for finishing,        the plant element for preparing acetone cyanohydrin comprising a        loop reactor with a cooler, the cooler comprising a cooling        region which can be flowed through and a cooling element.        Connected in fluid-conducting form means in the present context        that gases, liquids and gas-liquid mixtures or other        free-flowing substances can be conducted. In connection with        preferred embodiments of the cooler or of the cooling region and        also of the cooling element or of the cooling elements,        reference is made to the remarks on this subject in this text.

It is also preferred in accordance with the invention that the processaccording to the invention for preparing an alkyl methacrylate iseffected in an inventive apparatus.

The present invention also relates to a process for preparing polymersbased at least partly on alkyl methacrylates, comprising the steps of:

-   A1) preparing an alkyl methacrylate by a process according to the    invention;-   A2) polymerizing the alkyl methacrylate and optionally a comonomer;-   A3) working up the alkyl methacrylate.

Useful comonomers include all of those which are known to those skilledin the art and appear to be suitable, particular preference being givento free-radically polymerizable monomers. Among these, mention should bemade in particular of styrene, butyl acrylate or acrylonitrile, methylacrylate or ethyl acrylate.

Polymerization can be performed as a solution, bead, emulsion orsuspension polymerization, or else as a bulk polymerization. The polymeris worked up, for example, by precipitation of the solvent-comprisingpolymer in a nonsolvent for the polymer as a precipitant. For example, apolymer comprising acetone as the solvent and polymethyl methacrylate isprecipitated in a precipitant composed of methanol and water, separatedfrom the precipitant and dried.

In addition, the invention relates to the use of an ultrapure alkylmethacrylate obtainable by the process according to the invention infibres, films, coatings, moulding compositions, mouldings, papermakingauxiliaries, leather auxiliaries, flocculants and drilling additives aspreferred chemical products.

In addition, the invention relates to fibres, films, coatings, mouldingcompositions, mouldings, papermaking auxiliaries, leather auxiliaries,flocculants and drilling additives as preferred chemical products whichare based on a pure methacrylic ester obtainable by the processaccording to the invention.

Various process elements and plant parts will be illustratedhereinafter, which can in principle be combined with the presentinvention individually or as an ensemble of two or more of the processelements mentioned. In some cases, it may be advantageous when theprocess elements presented within the present text are combined with thepresent invention such that they are combined overall to give a processfor preparing esters of methacrylic acid or a process for preparingmethacrylic acid. However, it should also be pointed out thatadvantageous effects can usually also be achieved when the subjectmatter of the present invention as such is used in another field or onlycombined with some of the process elements presented here.

Preparation of Acetone Cyanohydrin

In this process element, acetone cyanohydrin is prepared by commonlyknown processes (see, for example, Ullmann's Enzyklopädie dertechnischen Chemie, 4th Edition, Volume 7). Frequently, the reactantsused are acetone and hydrocyanic acid. The reaction is an exothermicreaction. In order to counteract decomposition of the acetonecyanohydrin formed in this reaction, the heat of reaction is typicallyremoved by a suitable apparatus. The reaction can be conducted inprinciple as a batch process or as a continuous process; when acontinuous process is preferred, the reaction is frequently performed ina loop reactor which is fitted out appropriately.

A main feature of a method leading to the desired product in high yieldsis often that the reaction product is cooled at sufficient reaction timeand the reaction equilibrium is shifted in the direction of the reactionproduct. In addition, the reaction product is frequently admixed with anappropriate stabilizer to the advantage of the overall yield, in orderto prevent decomposition in the course of the later workup to give thestarting materials.

The mixing of the acetone and hydrocyanic acid reactants can inprinciple be effected in essentially any way. The method of mixingdepends in particular on whether a batchwise mode, for example in abatch reactor, or a continuous mode, for example in a loop reactor, isselected.

In principle, it may be advantageous when the acetone is fed into thereaction via a reservoir vessel which has a scrubber tower. Ventinglines which conduct waste air containing acetone and hydrocyanic acidcan thus be conducted, for example, through this reservoir vessel. Inthe scrubbing tower which is attached to the reservoir vessel, the wasteair escaping from the reservoir vessel can be scrubbed with acetone,which removes hydrocyanic acid from the waste air and recycles it intothe process. For this purpose, for example, some of the amount ofacetone introduced from the reservoir vessel into the reaction isconducted in the part-stream through a cooler, preferably through abrine cooler, into the top of the scrubbing tower and the desired resultis thus achieved.

Depending on the size of the amount of end products to be produced, itmay be advantageous to feed the acetone to the reaction from more thanjust one reservoir vessel. In this context, it is possible for each ofthe two or more reservoir vessels to bear a corresponding scrubbingtower. However, it is in many cases sufficient when only one of thereservoir vessels is equipped with a corresponding scrubbing tower. Inthis case, it is, however, often advisable for corresponding lines whichconduct waste air and can transport acetone and hydrocyanic acid to beconducted through this vessel or through this scrubbing tower.

The temperature of the acetone in the reservoir may in principle bewithin an essentially arbitrary range, provided that the acetone is inthe liquid state at the appropriate temperature. The temperature in thereservoir vessel is advantageously, however, about 0 to about 20° C.

In the scrubbing tower, the acetone used for scrubbing is cooled bymeans of an appropriate cooler, for example by means of a plate cooler,with brine to a temperature of about 0 to about 10° C. The temperatureof the acetone on entry into the scrubbing tower is thereforepreferably, for example, about 2 to about 6° C.

The hydrocyanic acid required in the reaction can be introduced into thereactor either in liquid or in gaseous form. It may, for example, becrude gas from the BMA process or from the Andrussow process.

The hydrogen cyanide can, for example, be liquefied, for example by theuse of an appropriate cooling brine. Instead of liquefied hydrocyanicacid, coking oven gas can be used. For example, hydrogencyanide-containing coking oven gases, after scrubbing with potash, arescrubbed continuously in countercurrent with acetone which contains 10%water, and the reaction to give acetone cyanohydrin can be carried outin the presence of a basic catalyst in two gas scrubbing columnsconnected in series.

In a further embodiment, a gas mixture comprising hydrogen cyanide andinert gases, especially a crude gas from the BMA process or from theAndrussow process, can be reacted with acetone in the presence of abasic catalyst and acetone cyanohydrin in a gas-liquid reactor.

In the process described here, preference is given to using a BMA crudegas or an Andrussow crude gas. The gas mixture resulting from theabovementioned customary processes for preparing hydrogen cyanide can beused as such or after an acid scrubbing. The crude gas from the BMAprocess, in which essentially hydrocyanic acid and hydrogen are formedfrom methane and ammonia, contains typically 22.9% by volume of HCN,71.8% by volume of H₂, 2.5% by volume of NH₃, 1.1% by volume of N₂, 1.7%by volume of CH₄. In the known Andrussow process, hydrocyanic acid andwater are formed from methane and ammonia and atmospheric oxygen. Thecrude gas of the Andrussow process, when oxygen is used as the oxygensource, contains typically 8% by volume of HCN, 22% by volume of H₂O,46.5% by volume of N₂, 15% by volume of H₂, 5% by volume of CO, 2.5% byvolume of NH₃ and 0.5% by volume each of CH₄ and CO₂.

When a non-acid-scrubbed crude gas from the BMA process or Andrussowprocess is used, the ammonia present in the crude gas frequently acts asa catalyst for the reaction. Since the ammonia present in the crude gasfrequently exceeds the amount required as a catalyst and can thereforelead to high losses of sulphuric acid used for stabilization, such acrude gas is often subjected to an acid scrubbing in order to eliminateammonia therefrom. When such an acid-scrubbed crude gas is used, it isthen necessary, however, to add a suitable basic catalyst to the reactorin a catalytic amount. In principle, known inorganic or organic basiccompounds may function as the catalyst.

Hydrogen cyanide in gaseous or in liquid form, or a gas mixturecomprising hydrogen cyanide, and acetone are fed continually to a loopreactor in the continuous mode. In this case, the loop reactor comprisesat least one means of feeding acetone or two or more such means, atleast one means for feeding liquid or gaseous hydrocyanic acid, or twoor more such means, and at least one means for feeding a catalyst.

Suitable catalysts are in principle any alkaline compounds, such asammonia, sodium hydroxide solution or potassium hydroxide solution,which can catalyse the reaction of acetone and hydrocyanic acid to giveacetone cyanohydrin. However, it has also been found to be advantageouswhen the catalyst used is an organic catalyst, especially an amine.Suitable examples are secondary or tertiary amines such as diethylamine,dipropylamine, triethylamine, tri-n-propylamine and the like.

A loop reactor useable in the process element described furthercomprises at least one pump, or two or more pumps, and at least onemixing apparatus, or two or more such mixing apparatuses.

Suitable pumps are in principle all pumps which are suitable forensuring the circulation of the reaction mixture in the loop reactor.

Suitable mixing apparatuses are both mixing apparatuses with mobileelements and so-called static mixers in which immobile flow resistancesare provided. In the case of use of static mixers, suitable examples arethose which allow an operational transfer of at least about 10 bar, forexample at least about 15 bar or at least about 20 bar under operatingconditions without significant restrictions in the functioning.Appropriate mixers may consist of plastic or metal.

Suitable plastics are, for example, PVC, PP; HDPE, PVDF, PVA or PTFE.Metal mixers may consist, for example, of nickel alloys, zirconium,titanium and the like. Likewise suitable are, for example, rectangularmixers.

The catalyst is added in the loop reactor preferably downstream of thepump and upstream of a mixing element present in the loop reactor. Inthe reaction described, catalysts are used, for example, in such anamount that the overall reaction is conducted at a pH of not more than8, in particular not more than about 7.5 or about 7. It may be preferredwhen the pH in the reaction varies within a range of about 6.5 to about7.5, for example about 6.8 to about 7.2.

Alternatively to the addition of the catalyst into the loop reactordownstream of the pump and upstream of a mixing apparatus, it is alsopossible in the process described to feed the catalyst into the loopreactor together with the acetone. In such a case, it may beadvantageous when appropriate mixing of acetone and catalyst is ensuredbefore feeding into the loop reactor. Appropriate mixing can beeffected, for example, by the use of a mixer with moving parts or by useof a static mixer.

When a continuous process in a loop reactor is selected as the operatingmode in the process described, it may be appropriate to examine thestate of the reaction mixture by instantaneous or continual analyses.This offers the advantage that, where appropriate, it is also possibleto react rapidly to changes in state in the reaction mixture.Furthermore, it is thus possible, for example, to meter in the reactantsvery precisely in order to minimize yield losses.

Corresponding analysis can be effected, for example, by sampling in thereactor loop. Suitable analysis methods are, for example, pHmeasurement, measurement of the exothermicity or measurement of thecomposition of the reaction mixture by suitable spectroscopic processes.

Especially within the context of conversion monitoring, quality aspectsand safety, it has been found to be useful to determine the conversionin the reaction mixture via the heat removed from the reaction mixtureand to compare it with the heat which is released theoretically.

In the case of suitable selection of the loop reactor, the actualreaction can in principle be effected within the tube systems arrangedwithin the loop reactor. Since the reaction, however, is exothermic, inorder to avoid yield loss, sufficient cooling and sufficient removal ofthe heat of reaction should be ensured. It has frequently been found tobe advantageous when the reaction proceeds within a heat exchanger,preferably within a tube bundle heat exchanger. Depending on the amountof product to be produced, the capacity of an appropriate heat exchangercan be selected differently. For industrial scale processes, heatexchangers having a volume of about 10 to about 40 m³ in particular havebeen found to be particularly suitable. The tube bundle heat exchangersused with preference are heat exchangers which have a tube bundle flowedthrough by liquid within a jacket flowed through by liquid. Depending onthe tube diameter, packing density, etc., the heat transfer between thetwo liquids can be adjusted appropriately. It is possible in principlein the process described to conduct the reaction to the effect that thereaction mixture is conducted through the heat exchanger in the tubebundle itself and the reaction takes place within the tube bundle, theheat being removed from the tube bundle into the jacket liquid.

However, it has likewise been found to be practicable and in many casesto be viable to conduct the reaction mixture through the jacket of theheat exchanger, while the liquid used for cooling is circulated withinthe tube bundle. It has in many cases been found to be advantageous whenthe reaction mixture is distributed within the jacket by means of flowresistances, preferably deflecting plates, to achieve a higher residencetime and better mixing.

The ratio of jacket volume to the volume of the tube bundle may,depending on the design of the reactor, be about 10:1 to about 1:10; thevolume of the jacket is preferably greater than the volume of the tubebundle (based on the contents of the tubes).

The heat removal from the reactor is adjusted with an appropriatecoolant, for example with water, such that the reaction temperature iswithin a corridor of about 25 to about 45° C., in particular about 30 toabout 38° C., in particular about 33 to about 35° C.

A product is removed continuously from the loop reactor. The product hasa temperature within the abovementioned reaction temperatures, forexample a temperature of about 35° C. The product is cooled by means ofone or more heat exchangers, especially by means of one or more plateheat exchangers. For example, brine cooling is used. The temperature ofthe product after cooling should be about 0 to 10° C., in particular 1to about 5° C. The product is preferably transferred into a storagevessel which has a buffer function. In addition, the product in thestorage vessel can be cooled further, for example, by constantlyremoving a part-stream from the storage vessel to a suitable heatexchanger, for example to a plate heat exchanger, or kept at a suitablestorage temperature. It is entirely possible that continued reaction cantake place in the storage vessel.

The product can be recycled into the storage vessel in any way inprinciple. However, it has been found to be advantageous in some casesthat the product is recycled by means of a system composed of one ormore nozzles into the storage vessel such that corresponding mixing ofthe stored product takes place within the storage vessel.

Product is also removed continuously from the storage vessel into astabilization vessel. The product is admixed there with a suitable acid,for example with H₂SO₄. This deactivates the catalyst and adjusts thereaction mixture to a pH of about 1 to about 3, in particular about 2. Asuitable acid is in particular sulphuric acid, for example sulphuricacid having a content of about 90 to about 105%, in particular of about93 to about 98% H₂SO₄.

The stabilized product is withdrawn from the stabilization vessel andtransferred into the purification stage. A portion of the stabilizedproduct withdrawn can be recycled, for example, into the stabilizationvessel such that sufficient mixing of the vessel is ensured by means ofa system composed of one or more nozzles.

ACH Workup

In a further process element which can be used in connection with thepresent invention, acetone cyanohydrin which has been obtained in apreceding stage, for example from the reaction of acetone withhydrocyanic acid, is subjected to a distillative workup. The stabilizedcrude acetone cyanohydrin is freed of low-boiling constituents by meansof a corresponding column. A suitable distillation process can beconducted, for example, by means of only one column. However, it islikewise possible, in an appropriate purification of crude acetonecyanohydrin, to use a combination of two or more distillation columns,also combined with a falling-film evaporator. In addition, two or morefalling-film evaporators or else two or more distillation columns may becombined with one another.

The crude acetone cyanohydrin comes from the storage to the distillationgenerally with a temperature of about 0 to about 15° C., for example atemperature of about 5 to about 10° C. In principle, the crude acetonecyanohydrin can be introduced directly into the column. However, it hasbeen found to be useful in some cases when the crude cool acetonecyanohydrin, by means of a heat exchanger, first takes up some of theheat of the product already purified by distillation. Therefore, in afurther embodiment of the process described here, the crude acetonecyanohydrin is heated by means of a heat exchanger to a temperature ofabout 60 to 80° C.

The acetone cyanohydrin is purified by distillation by means of adistillation column, preferably having more than 10 trays, or by meansof a battery of two or more corresponding suitable distillation columns.The column bottom is heated preferably with steam. It has been found tobe advantageous when the bottom temperature does not exceed atemperature of 140° C.; good yields and good purification have beenachieved when the bottom temperature is not greater than about 130° C.or not higher than about 110° C. The temperature data are based on thewall temperature of the column bottom.

The crude acetone cyanohydrin is fed to the column body in the upperthird of the column. The distillation is performed preferably underreduced pressure, preferably at a pressure of about 50 to about 900mbar, in particular about 50 to about 250 mbar and with good resultsbetween 50 and about 150 mbar.

At the top of the column, gaseous impurities, especially acetone andhydrocyanic acid, are removed, the removed gaseous substances are cooledby means of one heat exchanger or a battery of two or more heatexchangers. Preference is given here to using brine cooling with atemperature of about 0 to about 10° C. This gives the gaseousconstituents of the vapours the opportunity to condense. The firstcondensation stage may take place, for example, at standard pressure.However, it is equally possible and has been found to be advantageous insome cases when this first condensation stage is effected under reducedpressure, preferably at the pressure which prevails in the distillation.The condensate is passed on into a cooled collecting vessel andcollected there at a temperature of about 0 to about 15° C., inparticular at about 5 to about 10° C.

The gaseous compounds which do not condense in the first condensationstep are removed from the reduced pressure chamber by means of a vacuumpump. In principle, any vacuum pump can be used here. However, it hasbeen found to be advantageous in many cases to use a vacuum pump which,owing to its design, does not lead to the introduction of liquidimpurities into the gas stream. Preference is therefore given here, forexample, to using dry-running vacuum pumps.

The gas stream which escapes on the pressure side of the pump isconducted through a further heat exchanger which is preferably cooledwith brine at a temperature of about 0 to about 15° C. Constituentswhich condense here are likewise collected in the collecting vesselwhich already collects the condensates obtained under vacuum conditions.The condensation performed on the pressure side of the vacuum pump canbe effected, for example, by a heat exchanger, but also with a batteryof two or more heat exchangers arranged in series or in parallel.Gaseous substances remaining after this condensation step are removedand sent to any further utilization, for example a thermal utilization.

The collected concentrates may likewise be utilized further as desired.However, for economic reasons, it has been found to be extremelyadvantageous to recycle the condensates into the reaction for preparingacetone cyanohydrin. This is effected preferably at one or more pointswhich enable access to the loop reactor. The condensates may inprinciple have any composition provided that they do not disrupt thepreparation of the acetone cyanohydrin. In many cases, a predominantamount of the condensate will, however, consist of acetone andhydrocyanic acid, for example in a molar ratio of about 2:1 to about1:2, frequently in a ratio of about 1:1.

The acetone cyanohydrin obtained from the bottom of the distillationcolumn is first cooled to a temperature of about 40 to about 80° C. bythe cold crude acetone cyanohydrin fed in by means of a first heatexchanger. Subsequently, the acetone cyanohydrin is cooled to atemperature of about 30 to about 35° C. by means of at least one furtherheat exchanger and optionally stored intermediately.

Amidation

In a further process element as frequently provided in the preparationof methacrylic acid or of esters of methacrylic acid, acetonecyanohydrin is subjected to a hydrolysis. At different temperaturelevels after a series of reactions, this forms methacrylamide as theproduct.

The reaction is brought about in a manner known to those skilled in theart by a reaction between concentrated sulphuric acid and acetonecyanohydrin. The reaction is exothermic, which means that heat ofreaction is advantageously removed from the system.

The reaction here too can again be performed in a batchwise process orin continuous processes. The latter has been found to be advantageous inmany cases. When the reaction is performed in a continuous process, theuse of loop reactors has been found to be useful. The reaction can beeffected, for example, in only one loop reactor. However, it may beadvantageous when the reaction is performed in a battery of two or moreloop reactors.

In the process described, a suitable loop reactor has one or more feedpoints for acetone cyanohydrin, one or more feed points for concentratedsulphuric acid, one or more gas separators, one or more heat exchangersand one or more mixers, and often a pump as a conveying means.

The hydrolysis of acetone cyanohydrin with sulphuric acid to givemethacrylamide is, as already described, exothermic. The heat ofreaction which arises in the reaction must, however, at least largely bewithdrawn from the system, since the yield falls with increasingtemperature in the reaction. It is possible in principle to achieve arapid and comprehensive removal of the heat of reaction with appropriateheat exchangers. However, it may also be disadvantageous to cool themixture too greatly, since a sufficient heat transfer is required forappropriate exchange at the heat exchangers. Since the viscosity of themixture rises greatly with falling temperature, the circulation in andthe flow through the loop reactor is firstly complicated, and sufficientremoval of the reaction energy from the system can secondly no longer beensured.

In addition, excessively low temperatures in the reaction mixture canlead to a crystallization of constituents of the reaction mixture at theheat exchangers. This further worsens the heat transfer, as a result ofwhich a clear yield reduction can be detected. In addition, the loopreactor cannot be charged with the optimal amounts of reactants, suchthat the efficiency of the process suffers overall.

In one embodiment of the process, a portion, preferably about two thirdsto about three quarters, of the volume flow rate from a stream ofacetone cyanohydrin is introduced into a first loop reactor. A firstloop reactor preferably has one or more heat exchangers, one or morepumps, one or more mixing elements and one or more gas separators. Thecirculation streams which pass through the first loop reactor arepreferably in the range of about 50 to 650 m³/h, preferably in a rangeof 100 to 500 m³/h and more preferably in a range of about 150 to 450m³/h. In an at least one further loop reactor which follows the firstloop reactor, the circulation streams are preferably in a range of about40 to 650 m³/h, preferably in a range of 50 to 500 m³/h and morepreferably in a range of about 60 to 350 m³/h. Moreover, a preferredtemperature difference over the heat exchangers is about 1 to 20° C.,particular preference being given to about 2 to 7° C.

The acetone cyanohydrin can in principle be fed into the loop reactor atany point. However, it has been found to be advantageous when the feedis into a mixing element, for example into a mixer with moving parts ora static mixer, or at a well-mixed point. The sulphuric acid isadvantageously fed in upstream of the acetone cyanohydrin addition.Otherwise, it is, however, likewise possible to feed the sulphuric acidinto the loop reactor at any point.

The ratio of the reactants in the loop reactor is controlled such thatan excess of sulphuric acid is present. The excess of sulphuric acid is,based on the molar ratio of the constituents, about 1.8:1 to about 3:1in the first loop reactor and about 1.3:1 to about 2:1 in the last loopreactor.

In some cases, it has been found to be advantageous to perform thereaction in the loop reactor with such an excess of sulphuric acid. Thesulphuric acid may serve here, for example, as a solvent and keep theviscosity of the reaction mixture low, which can ensure a higher removalof heat of reaction and a lower temperature of the reaction mixture.This can entail significant yield advantages. The temperature in thereaction mixture is about 90 to about 120° C.

The heat removal is ensured by one or more heat exchangers in the loopreactor. It has been found to be advantageous when the heat exchangershave a suitable sensor system for controlling the cooling performance inorder to prevent excessively great cooling of the reaction mixture forthe aforementioned reasons. For example, it may be advantageous tomeasure the heat transfer in the heat exchanger or in the heatexchangers point by point or continuously and to adjust the coolingperformance of the heat exchangers thereto. This can be done, forexample, via the coolant itself. It is equally possible to achieveappropriate heating of the reaction mixture by corresponding variationof the addition of the reactants and by the generation of more heat ofreaction. A combination of the two possibilities is also conceivable.The loop reactor should additionally have at least one gas separator.One method is to withdraw product formed continuously from the loopreactor via the gas separator. Another method is thus to withdraw thegases formed in the reaction from the reaction chamber. The gas formedis mainly carbon monoxide. The product withdrawn from the loop reactoris preferably transferred into a second loop reactor. In this secondloop reactor, the reaction mixture comprising sulphuric acid andmethacrylamide, as has been obtained by the reaction in the first loopreactor, is reacted with the remaining part-stream of acetonecyanohydrin. In this case, the excess of sulphuric acid from the firstloop reactor, or at least some of the excess sulphuric acid, reacts withthe acetone cyanohydrin to form further methacrylamide. The performanceof the reaction in two or more loop reactors has the advantage that,owing to the sulphuric acid excess in the first loop reactor, thepumpability of the reaction mixture and hence the heat transfer andultimately the yield are improved. In turn, at least one mixing element,at least one heat exchanger and at least one gas separator are arrangedin the second loop reactor. The reaction temperature in the second loopreactor is likewise about 90 to about 120° C.

The problem of the pumpability of the reaction mixture, of heat transferand of a minimum reaction temperature occurs in every further loopreactor just as it does in the first. Therefore, the second loop reactortoo advantageously has a heat exchanger whose cooling performance can becontrolled by an appropriate sensor system.

The acetone cyanohydrin is again fed in a suitable mixing element,preferably into a static mixer or at a well-mixed point.

The product is withdrawn from the separator, especially gas separator,of the second loop reactor and heated to a temperature of about 130 toabout 180° C. to complete the reaction and to form the methacrylamide.

The heating is preferably performed such that the maximum temperature isattained only for a minimum period, for example for a time of about oneminute to about 30 minutes, in particular for a time of about two toabout eight or about three to about five minutes. This can in principlebe effected in any apparatus for achieving such a temperature for such ashort period. For example, the energy can be supplied in a conventionalmanner by electrical energy or by steam. However, it is equally possibleto supply the energy by means of electromagnetic radiation, for exampleby means of microwaves.

In various cases, it has been found to be advantageous when the heatingstep is effected in a heat exchanger with two-stage or multistagearrangement of tube coils which may preferably be present in an at leastdouble, opposite arrangement. This heats the reaction mixture rapidly toa temperature of about 130 to 180° C.

The heat exchanger can be combined, for example, with one or more gasseparators. For example, it is possible to conduct the reaction mixturethrough a gas separator after it leaves the first tube coil in the heatexchanger. This can remove, for example, gaseous components formedduring the reaction from the reaction mixture. It is equally possible totreat the reaction mixture with a gas separator after it leaves thesecond coil. It may additionally be found to be advantageous to treatthe reaction mixture with a gas separator both after it leaves the firsttube coil and after it leaves the second tube coil.

The amide solution thus obtainable generally has a temperature of morethan 100° C., typically a temperature of about 130 to 180° C.

The gaseous compounds obtained in the amidation can in principle bedisposed of in any way or sent to further processing. However, it may beadvantageous in some cases when the appropriate gases are combined in atransport line in such a way that they are optionally pressurized eithercontinuously or as required, for example with steam pressure, and canthus be transported further.

Esterification

A further step which constitutes a process element and can be used inthe present invention in connection with the process according to theinvention is a hydrolysis of methacrylamide to methacrylic acid and itssubsequent esterification to methacrylic esters. This reaction can beperformed in one or more heated, for example steam-heated, tanks.However, it has in many cases been found to be advantageous when theesterification is performed in at least two successive tanks, but, forexample, also in three or four or more successive tanks. In this case, asolution of methacrylamide is introduced into the tank or into the firsttank of a battery of tanks comprising two or more tanks.

It is frequently preferred to perform a corresponding esterificationreaction with a battery of two or more tanks. Reference will thereforebe made hereinafter exclusively to this variant.

In the process described here, it is possible, for example, to feed anamide solution as obtainable from the amidation reaction described hereinto a first tank. The tank is heated, for example, with steam. Theamide solution supplied generally has an elevated temperature, forexample a temperature of about 100 to about 180° C., essentiallycorresponding to the exit temperature of the amide solution from theamidation reaction presented above. An alkanol is also fed to the tanks,which can be used for the esterification.

Suitable alkanols here are in principle any alkanols having 1 to about 4carbon atoms, which may be linear or branched, saturated or unsaturated,particular preference being given to methanol. These alkanols maylikewise be used together with methacrylic esters, which is the caseespecially in transesterifications.

The tank is also charged with water, so that there is a total waterconcentration in the tank of about 13 to about 26% by weight, inparticular about 18 to about 20% by weight.

The amount of amide solution and of alkanol is controlled such that atotal molar ratio of amide to alkanol of about 1:1.4 to about 1:1.6exists. The alkanol can be distributed over the tank battery such thatthe molar ratio in the first reactor is about 1:1.1 to about 1:1.4 and,in the subsequent reaction stages, based on the total amide stream,molar ratios of about 1:0.05 to about 1:0.3 are established. The alkanolfed into the esterification may be composed of “fresh alkanol” andalkanol from recycling streams of the workup stages and, if required,also of recycling streams of the downstream processes of the productionsystem.

The first tank can be charged with water in principle such that water isfed to the tank from any source, provided that this water has noingredients which might adversely affect the esterification reaction orthe downstream process stages. For example, demineralized water orspring water can be fed to the tank. However, it is likewise possible tofeed a mixture of water and organic compounds to the tank, as obtained,for example, in the purification of methacrylic acid or methacrylicesters. In a preferred embodiment of the process presented here, thetank is charged at least partly with a mixture of water and such organiccompounds.

When a battery of two or more tanks is used in the esterificationreaction, the gaseous substances formed, especially the methacrylicesters, can in principle be drawn off individually from each tank andfed to a purification. However, it has been found in some cases to beadvantageous when, in a battery of two or more tanks, the gaseousproducts from the first tank are first fed into the second reactionvessel without the gaseous compounds from the first tank being feddirectly to a purification. This procedure offers the advantage that thefrequently high evolution of foam in the first tank need not becounteracted by complicated defoaming apparatus. In the case of passageof the gaseous substances from the first tank into the second tank, thefoam which has been formed in the first tank and may have been entrainedalso enters the reaction chamber of the second tank in a simple manner.Since the foam formation there is generally significantly lower, thereis no need to use defoaming apparatus.

The second tank arranged downstream of a first tank then firstly takesup the overflow of the first tank; secondly, it is fed with the gaseoussubstances formed in the first tank or which are present in the firsttank. The second tank and any following tanks are likewise charged withmethanol. It is preferred here that the amount of methanol decreases byat least 10% from tank to tank, based in each case on the precedingtank. The water concentration in the second tank and in the furthertanks may differ from that of the first tank; the concentrationdifferences are, though, often small.

The vapours formed in the second tank are removed from the tank andintroduced into the bottom of a distillation column.

When the esterification is performed with a battery of three or moretanks, the overflow of the second tank is transferred in each case intoa third tank, and the overflow of the third tank, if appropriate, into afourth tank. The further tanks are likewise steam-heated. Thetemperature in tanks 3 and, if appropriate, 4 is preferably adjusted toabout 120 to about 140° C.

The vapours escaping from the tanks are passed into a distillationcolumn, this preferably being effected in the lower region of thedistillation column. The vapours comprise an azeotropic mixture ofcarrier steam, methacrylic esters and alkanol and, depending on thealkanol used, have a temperature of about 60 to about 120° C., forexample about 70 to about 90° C., when methanol is used. In thedistillation column, the methacrylic ester is separated in gaseous formfrom the vapour constituents which boil at higher temperatures. Thehigh-boiling fractions (mainly methacrylic acid, hydroxyisobutyricesters and water) are recycled into the first reaction tank. Themethacrylic ester formed is drawn off at the top of the column andcooled by means of a heat exchanger or a battery of two or more heatexchangers. It has been found to be useful in some cases when themethacrylic ester is cooled by means of at least two heat exchangers, inwhich case a first heat exchanger with water performs the condensationand a cooling to a temperature of about 60 to about 30° C., while asecond brine-cooled heat exchanger undertakes a cooling to about 5 toabout 15° C. A part-stream from the water-cooled condensate can beintroduced as reflux to the columns for concentration control in thecolumn. However, it is equally possible to cool the methacrylic esterformed by means of a battery of more than two heat exchangers. In thiscase, it is possible, for example, first to undertake a cooling by meansof two water-cooled heat exchangers connected in series and then toachieve a further cooling by means of an appropriate brine-cooled heatexchanger.

For example, in the process presented here, the methacrylic ester formedcan be cooled in the gaseous state by means of a first heat exchangerwith water cooling. Both condensed and uncondensed substances are thenpassed on into a second heat exchanger, where a further condensation bymeans of water cooling takes place. At this point, for example, gaseoussubstances can then be transferred into a separate brine-cooled heatexchanger. The condensate in this brine-cooled heat exchanger is thenintroduced into the distillate stream, while the remaining gaseoussubstances can be utilized further or sent to disposal. The methacrylicester condensate from the second water-cooled heat exchanger is thencooled in a water-cooled or brine-cooled heat exchanger to a temperatureof less than 15° C., preferably about 8 to about 12° C. This coolingstep can lead to the methacrylic ester formed having a significantlylower content of formic acid than would be the case without thecorresponding cooling step. The cooled condensate is then transferred toa phase separator. Here, the organic phase (methacrylic ester) isseparated from the aqueous phase. The aqueous phase which, as well aswater, may also have a content of organic compounds, especially alkanol,from the distillation step may in principle be used further as desired.However, as already described above, it may be preferred to recycle thismixture of water and organic compounds back into the esterificationprocess by feeding it into the first reaction tank.

The removed organic phase is fed into a scrubber. There, the methacrylicester is scrubbed with demineralized water. The separated aqueous phasewhich comprises a mixture of water and organic compounds, especiallyalkanol, can in turn in principle be used further as desired. However,it is advantageous for economic reasons to recycle this aqueous phaseback into the esterification step by feeding it, for example, into thefirst tank.

Since methacrylic esters have a strong tendency to polymerize, it is inmany cases advantageous when care is taken in the esterification ofmethacrylic acid that such a polymerization is prevented.

In plants for preparing methacrylic acid or methacrylic esters,polymerization often takes place when methacrylic acid or methacrylicester firstly have a low flow rate, so that local calm zones can form,in which contact lasting over a long period between methacrylic acid ormethacrylic ester and a polymerization initiator can be established,which can subsequently lead to polymerization.

In order to prevent such polymerization behaviour, it may beadvantageous to optimize the substance flow to the effect that, firstly,the flow rate of the methacrylic ester or of the methacrylic acid issufficiently high at substantially all points in the system that thenumber of calm zones is minimized. Furthermore, it may be advantageousto admix the stream of methacrylic acid or methacrylic ester withsuitable stabilizers such that polymerization is largely suppressed.

For this purpose, the streams in the process presented here can inprinciple be admixed with stabilizers such that a minimum level ofpolymerization takes place in the system itself. To this end, the partof the plant in particular in which the methacrylic acid or themethacrylic ester is present in high concentration during or after thedistillation is supplied with appropriate stabilizers.

For example, it has been found to be viable to supply a stabilizer atthe top of the distillation column to the stream of methacrylic esterdrawn off there. Furthermore, it has been found to be advantageous toflush those parts of the plant in which methacrylic acid or methacrylicester is circulated with a temperature of more than about 20° C.,preferably with a temperature in the range of about 20 to about 120° C.,with a solution of stabilizer in methacrylic ester. For example, some ofthe condensate obtained in the heat exchangers, together with a suitablestabilizer, is recycled into the top of the distillation column suchthat the column top, on its interior, is sprayed constantly withstabilized methacrylic ester or stabilized methacrylic acid. This ispreferably done in such a way that no calm zones can form in the top ofthe column, at which there is a risk of polymerization of methacrylicacid or methacrylic ester. The heat exchangers themselves maycorrespondingly likewise be charged with a stabilized solution ofmethacrylic acid or methacrylic ester in such a way that no calm zonescan form here either.

It has also been found to be advantageous in the process presented herewhen, for example, the offgases comprising carbon monoxide frompreceding processes, especially from the amidation step, are passedthrough the esterification plant together with steam. In this way, thegas mixture is once again purified to remove compounds which can beremoved in solid or in liquid form. Secondly, these are collected at acentral point and can be sent to further utilization or disposal.

The methacrylic ester obtained or the MMA obtained in the esterificationand the subsequent prepurification, or the methacrylic acid obtained,are subsequently sent to a further treatment. The esterification resultsin dilute sulphuric acid as the remaining residual substance, which canlikewise be sent to a further utilization.

Prepurification of the Ester or of the Acid

In the process presented here, the subject matter of the presentinvention can also be used in connection with a process for prepurifyingmethacrylic acid or methacrylic ester, as described in the processelement which follows. For instance, in principle, crude methacrylicacid or a crude methacrylic ester can be subjected to a furtherpurification in order to arrive at a very pure product. Such apurification which constitutes a further process element can, forexample, be in one stage. However, it has been found to be advantageousin many cases when such a purification comprises at least two stages, inwhich case the low-boiling constituents of the product are removed in afirst prepurification as described here. To this end, crude methacrylicester or crude methacrylic acid is transferred first into a distillationcolumn in which the low-boiling constituents and water can be removed.To this end, the crude methacrylic ester is sent to a distillationcolumn, in which case the addition is performed, for instance, in theupper half of the column. The column bottom is heated with steam, forexample, in such a way that a wall temperature of about 50 to about 120°C. is achieved. The purification is performed under reduced pressure.The pressure within the column in the case of the ester is preferablyabout 100 to about 600 mbar. The pressure within the column in the caseof the acid is preferably about 40 to about 300 mbar.

At the top of the column, the low-boiling constituents are removed. Inparticular, these may, for example, be ether, acetone and methylformate. The vapours are then condensed by means of one or more heatexchangers. For example, it has been found to be useful in some casesfirst to perform a condensation by means of two water-cooled heatexchangers connected in series. However, it is equally possible to useonly one heat exchanger at this point. The heat exchangers arepreferably operated in an upright state to increase the flow rate and inorder to prevent the formation of stationary phases, preference beinggiven to obtaining maximum wetting. Connected downstream of thewater-cooled heat exchanger or the water-cooled heat exchangers may be abrine-cooled heat exchanger, but it is also possible to connect abattery of two or more brine-cooled heat exchangers downstream. In thebattery of heat exchangers, the vapours are condensed, provided withstabilizer and, for example, fed to a phase separator. Since the vapoursmay also contain water, any aqueous phase which occurs is disposed of orsent to a further utilization. An example of a possible furtherutilization is recycling into an esterification reaction, for exampleinto an esterification reaction as has been described above. In thiscase, the aqueous phase is preferably recycled into the firstesterification tank.

The removed organic phase is fed as reflux into the top of the column.Some of the organic phase can in turn be used to spray the tops of theheat exchangers and the top of the column. Since the removed organicphase is a phase which has been admixed with stabilizer, it is thuspossible firstly to effectively prevent the formation of calm zones.Secondly, the presence of the stabilizer brings about furthersuppression of the polymerization tendency of the vapours removed.

The condensate stream obtained from the heat exchangers is additionallypreferably admixed with demineralized water in such a way thatsufficient separating action can be achieved in the phase separator.

The gaseous compounds which remain after the condensation in the heatexchanger battery may, preferably by means of steam ejectors asreduced-pressure generators, be subjected once again to a condensationby means of one or more further heat exchangers. It has been found to beadvantageous for economic reasons when such a postcondensation condensesnot only the gaseous substances from the prepurification. For example,it is possible to feed further gaseous substances to such apostcondensation, as obtained from the main purification of methacrylicesters. The advantage of such a procedure lies, for example, intransferring such a proportion of methacrylic ester which has not beencondensed in the main purification stage once more via the phaseseparator into the purification column in the prepurification. It isthus ensured, for example, that a maximization of yield can take placeand minimum losses of methacrylic esters occur. Moreover, the suitableselection of the design and the operation of these further heatexchangers allows the composition of the offgas leaving these heatexchangers, especially the content of low boilers, to be adjusted.

Owing to the feeding of water in the prepurification of the methacrylicester, the water content in the esterification and the concentration oflow-boiling constituents in the crude methyl methacrylate overall canrise continuously. In order to prevent this, it may be advantageous todischarge some of the water fed to the system out of the system,preferably continuously. This discharge can in principle be effected,for example, in an order of magnitude in which water is fed to thesystem in the prepurification. The aqueous phase separated out in thephase separator typically has a content of organic constituents. It maytherefore be advantageous to feed this water to a form of disposal whichutilizes this content of organic substances.

For example, it may be advantageous when water thus contaminated withorganic substances is fed to the combustion chamber in a sulphuric acidcleavage process. Owing to the oxidizable constituents, its calorificvalue can still be utilized at least partly. In addition, a possiblyexpensive disposal of the water contaminated with organic substances isthus often avoided.

Fine Purification of the Methacrylic Ester

For the fine purification of the methacrylic ester, the crudeprepurified methacrylic ester is subjected to another distillation. Thisfrees the crude methacrylic ester of its high-boiling constituents withthe aid of a distillation column to obtain a pure methacrylic ester. Tothis end, the crude methacrylic ester is introduced into a distillationcolumn, sometimes into the lower half, in a manner known to thoseskilled in the art.

The distillation column can in principle correspond to any design whichappears to be suitable to those skilled in the art. However, it has beenfound to be advantageous in many cases for the purity of the resultingproduct when the distillation column is operated with one or morepackings which correspond approximately to the following requirements:

Firstly, just like in the other lines flowed through by methacrylicester, a minimum level of so-called “dead spaces” should form in thecolumns. The dead spaces lead to a comparatively long residence time ofthe methacrylic esters, which promotes their polymerization. This inturn leads to expensive production shutdowns and cleaning of theappropriate parts blocked with polymer. One way of countering theformation of dead spaces is, both by design and by a sufficientoperating mode of the columns, to always load them with a sufficientamount of liquid, so that constant flushing of the columns andespecially of the column internals such as packings is achieved. Forinstance, the columns may have spray devices which are designed for thespraying of the column internals. In addition, the column internals maybe connected to one another such that barely any dead spaces, or betternone at all, form. To this end, the column internals may be connected toone another or to the column via interrupted adhesion seams. Suchadhesion seams have at least about 2, preferably at least about 5 andmore preferably at least about 10 interruptions for 1 m of adhesion seamlength. The length of these interruptions may be selected such that theymake up at least about 10%, preferably at least about 20% and morepreferably at least about 50%, but generally not more than 95% of theadhesion seam length. Another design measure may be that, in theinternal regions of the column, especially those which come into contactwith the methacrylic esters, less than about 50%, preferably less thanabout 25% and more preferably less than about 10% of all surfaces,especially of column internals, run horizontally. For example, the stubswhich open into the interior of the column may be configured conicallyor with oblique surfaces. Another measure may consist in keeping theamount of liquid methacrylic ester present in the column bottom as lowas possible during the operation of the column, and secondly inpreventing overheating of this amount in spite of moderate temperaturesand large evaporation surfaces during the evaporation. It may beadvantageous in this context that the amount of liquid in the columnbottom makes up in the range of about 0.1 to 15% and preferably about 1to 10% of the total amount of methacrylic ester in the column. Themeasures proposed in this paragraph may also find use in thedistillation of methacrylic acid.

In the purification of the methacrylic ester, its high-boilingconstituents are separated from the product by distillation. To thisend, the column bottom is heated with steam. The bottom temperature ispreferably about 50 to about 80° C., in particular about 60 to about 75°C., with wall temperature of less than about 120° C.

The material obtained in the column bottom is preferably removedcontinuously and cooled by means of a heat exchanger or a battery ofseveral heat exchangers to a temperature in a range of about 40 to about80° C., preferably about 40 to about 60° C. and more preferably in arange of about 50 to 60° C.

This material, which comprises predominantly methacrylic ester,hydroxyisobutyric ester, methacrylic acid and stabilizer components, issubsequently, via a storage vessel, for example, disposed of or sent toanother use. It has been found to be advantageous in many cases when thematerial obtained in the column bottom is recycled into theesterification reaction. For example, the material from the columnbottom is recycled into the first esterification tank. This gives riseto the advantage that, with a view to a very economically viable methodand a very high yield, relatively high-boiling compounds present in thecolumn bottoms are recycled into the esterification reaction.

At the top of the column, the methacrylic ester purified by distillationis withdrawn and cooled by means of a heat exchanger or a battery of twoor more heat exchangers. The heat of the vapours can be removed by meansof water-cooled heat exchangers or by means of brine-cooled heatexchangers or by means of a combination of the two. It has been found tobe useful in some cases when the vapours from the distillation columnare transferred into two or more heat exchangers connected in parallel,which are operated by means of water cooling. The uncondensed fractionsfrom the water-cooled heat exchangers can, for example, be introducedinto a brine-cooled heat exchanger or a battery of two or morebrine-cooled heat exchangers, which may be arranged in series or inparallel. The condensates obtainable from the heat exchangers areintroduced into a collecting vessel and sent to a buffer vessel by meansof a pump via a further heat exchanger or a battery of two or morefurther heat exchangers. The condensate stream is cooled, for example,by means of a battery of one or two water-cooled heat exchangers and oneor two brine-cooled heat exchangers down to a temperature in a range ofabout 0 to about 20° C., preferably about 0 to about 15° C. and morepreferably in a range of about 2 to 10° C.

A part-stream is withdrawn from the condensate stream and is recycledinto the distillation column via the top of the column. The condensatestream can be fed into the top of the column in principle in any way,for example via distributors. However, it may be advantageous when aportion of the condensate stream is fed into the vapour line above thetop of the column, for example sprayed in. It is also preferred thatthis feeding also introduces stabilizer into the top of the column.

A further part-stream of the condensate intended for recycling into thecolumn can, for example, be branched off into the vapour line beforeintroduction and be introduced directly into the top of the column. Heretoo, it is preferred that this feeding introduces stabilizer into thetop of the column. The introduction into the top of the column can bedone, for example, in such a way that the interior of the top of thecolumn is sprayed with the condensate such that no calm zones can formin the top of the column at which the methacrylic ester can polymerize.It may additionally be advantageous to add a stabilizer for preventingpolymerization to a condensate part-stream which is recycled into thecolumn. This can be done, for example, by adding an appropriate amountof polymerization inhibitor as stabilizer to the condensate part-streamintended for spraying of the top of the column. It has been found to beadvantageous in some cases when the condensate part-stream, after theaddition of the stabilizer but before entry into the top of the column,passes through a suitable mixing apparatus, preferably a static mixer,in order to achieve very uniform distribution of the stabilizer in thecondensate part-stream.

The uncondensable gaseous substances which are obtained in thepurification process are, for example, sent to disposal.

The crude product present in the buffer vessel is kept with the aid of abrine cooler at a temperature of about 0 to about 20° C., preferablyabout 0 to about 15° C. and more preferably in a range of about 2 to 10°C.

In order to remove any further impurities from the product and to arriveat ultrapure alkyl methacrylates, the product can also be subjected toan absorptive purification stage. It has been found to be useful, forexample, when the pure product as a whole, or at least a portion of thepure product, is purified further with the aid of a molecular sieve.Particularly acidic impurities, especially formic acid formed in thepreparation process, can thus be removed in a simple manner from theproduct stream. It has additionally been found to be useful in somecases when the product stream, after passing through the adsorptivepurification stage, also passes through one or more filters in order toremove any solids present in the product.

The streams obtained in the workup comprise predominantly polymerizablecompounds. In order to, as already described more than once in thistext, prevent the formation of calm zones, it has been found to beadvantageous in the case of the process described here too when theparts of the plant which come into contact with methacrylic ester areconstantly flowed over with methacrylic ester. In a further embodimentof the process presented here, a part-stream of methacrylic ester istherefore withdrawn downstream of the buffer vessel but upstream of theadsorptive purification stage in order to be flushed over the topregions of those heat exchangers which take up the vapours stemming fromthe distillation column.

The product obtained in the purification stage is subsequently withdrawnfrom the purification stage with a temperature in a range of about −5 toabout 20° C., preferably about 0 to about 15° C. and more preferably ina range of about 2 to 10° C.

Stripping of the Consumed Acid

In the process presented here, it may be advisable, for example, in afurther process element, to subject the consumed sulphuric acid obtainedin the process to a purification in order to subsequently recycle itback into the process. In this case, for example, a stream comprisingconsumed sulphuric acid, as can be obtained from the esterification, canbe contacted with steam in a flotation vessel. As this is done, at leastsome of the solids present can be deposited on the surface of theliquid, and these deposited solids can be separated out. The vapours aresubsequently condensed in a heat exchanger, preferably with watercooling, cooled and recycled into the esterification reaction.

It has been found to be advantageous in some cases when corrosion isprevented in the heat exchangers and the cooling action is improvedfurther by introducing a mixture of water and organic compounds, asobtained by scrubbing in the course of the esterification in thepurification of the methacrylic ester prepared, into the heat exchangersin such a way that the tops of the heat exchangers are sprayed with thismixture. In addition to the corrosion-reducing action and the cooling ofthe acid in the heat exchanger, this procedure has a further advantage.Material which stems from the esterification (a mixture of water andpredominantly methanol) is recycled into the esterification processtogether with the methacrylic acid and methacrylic ester stemming fromexactly this process. In the stripper, the above-described flotationaffords mixtures of acid and solids. After their removal, these are sentto any further use or to disposal. It is possible, for example, toincinerate the resulting mixture in a cleavage plant and hence to obtainsulphuric acid again and in order to recover some of the energy used inthe process.

The uncondensable gaseous compounds obtained in the stripping are sentto any further use or disposed of.

The plant described here for removing solids from the consumed acid andfor recycling material from the esterification process into exactly thisprocess can also be performed, for example, twice for reasons ofoperational reliability. For instance, the two or more flotation vesselscan be used offset in time. Since solids can settle out in thesevessels, it is advantageous to remove them when the particular flotationvessel is not being used.

The aforementioned will now be illustrated in detail with reference tononlimiting drawings and examples. The schematic drawings show:

FIG. 1: a plant system for preparing and processing methacrylic acid ormethyl methacrylate,

FIG. 2: a plant for preparing acetone cyanohydrin,

FIG. 3: a workup plant for acetone cyanohydrin,

FIG. 4: an amidation plant,

FIG. 5: an esterification plant,

FIG. 6: a plant for prepurifying the ester,

FIG. 7: a fine purification plant for the ester,

FIG. 8: a heat exchanger as part of the plant for preparing acetonecyanohydrin.

FIG. 1 shows the preferred elements of a plant system 1 for preparingmethacrylic acid or methacrylic esters and their further processingproducts. The plant system 1 has various plants connected to one anotherusually in a fluid-conducting manner as elements of this system. Thisplant system includes acetone cyanohydrin preparation 20, followed byacetone cyanohydrin workup 30, followed by an amidation 40, followed byan esterification/hydrolysis 50/50 a, followed by a workup for ester ormethacrylic acid 60, followed in turn by a fine purification 70, afterwhich the ester, usually methyl methacrylate, or methacrylic acid ispresent. The pure ester/pure acid thus obtained can be sent to a furtherprocessing plant 80. Useful further processing plants 80 include inparticular polymerization apparatus and reactors for further organicreactions. In the polymerization reactors, polymethacrylates can beprepared, and, in the reactors for organic reactions, the pure monomersobtained here can be converted to further organic compounds. The furtherprocessing plant or the further processing plants 80 is/are followed bya finishing 90. When the further processing products are polymers ofmethacrylic acid or methacrylic esters, especially methyl methacrylate,they are processed further to give fibres, moulding compositions,especially granules, films, slabs, automobile parts and other mouldingsby suitable equipment such as extruders, blown-film extruders,injection-moulding machines, spinneret dies and the like. In addition,the plant system 1 in many cases comprises a sulphuric acid plant 100.For this plant, all sulphuric acid plants which appear to be suitablefor this purpose to the person skilled in the art are useful inprinciple. Reference is made in this context, for example, to Chapter 4,page 89 ff. in “Integrated Pollution Prevention and Control—DraftReference Document on Best Available Techniques for the Manufacture ofLarge Volume Inorganic Chemicals—Amino Acids and Fertilizers” obtainablevia the European Commission. The sulphuric acid plant 10 is connected toa series of other plants. For instance, the acetone cyanohydrinpreparation 20 is supplied with concentrated sulphuric acid via asulphuric acid line 2. Moreover, a further sulphuric acid line 3 existsbetween the sulphuric acid plant 100 and the amidation 40. The dilutesulphuric acid also referred to as “Spent Acid” from the esterification50 (hydrolysis 50 a) is transferred to the sulphuric acid plant 100through the lines for spent sulphuric acid 4 and 5. In the sulphuricacid plant 100, the dilute sulphuric acid can be worked up. The workupof the dilute sulphuric acid can be effected, for example, as describedin WO 02/23088 A1 or WO 02/23089 A1. In general, the plants aremanufactured from the materials which are familiar to those skilled inthe art and appear to be suitable for the particular stresses. Usually,the material is stainless steel which must in particular haveexceptional acid resistance. The regions of the plants which areoperated with sulphuric acid and especially with concentrated sulphuricacid are additionally lined and protected with ceramic materials orplastics. In addition, the methacrylic acid obtained in the methacrylicacid plant 50 a can be fed via a methacrylic acid line 6 to theprepurification 60. It has also been found to be useful to add astabilizer indicated with “S” in the acetone cyanohydrin preparation 20,the amidation 40, the esterification 50, the hydrolysis 50 a, theprepurification 60 and also the end purification 70.

In the acetone cyanohydrin preparation 20 shown in FIG. 2, the acetoneis provided in an acetone vessel 21 and the hydrocyanic acid in ahydrocyanic acid vessel 22. The acetone vessel 21 has a scrubbing tower23 which, in its upper region, has one or more cooling elements 24. Aseries of offgas lines 25 which stem from various plants in the plantsystem 1 open into the scrubbing tower 23. The acetone is fed into aloop reactor 26 via the acetone feed 27 and the hydrocyanic acid via thehydrocyanic acid feed 28. Downstream of the hydrocyanic acid feed 28 isdisposed a pump 29, followed in turn by a catalyst feed 210 which isfollowed by a static mixer 211. This is followed by a heat exchanger 212which has a series of flow resistances 213 and at least one cooling line214. In the loop reactor 26, the reaction mixture consisting of acetone,hydrocyanic acid and catalyst is conducted in a circuit to aconsiderable degree, which is indicated by bold lines. From the heatexchanger 212, the reaction mixture is conducted via the flowresistances along the cooling lines 214, and a portion of thecirculation stream is passed into a further heat exchanger 215 to whichis connected a collecting vessel 216 in which a nozzle 217 is present aspart of a cooling circuit 218 with a heat exchanger 219, which keeps thereaction product firstly in motion and secondly cool. Via an outlet 220which follows the collecting vessel 216, a stabilizer vessel 221 isattached, into which a sulphuric acid feed 222 opens and from which thecrude acetone cyanohydrin is conducted through the outlet 223 into theacetone cyanohydrin workup 30.

In FIG. 3, coming from the cyanohydrin preparation 20, the outlet 223opens into a heat exchanger 31 in which the stream coming from thecyanohydrin preparation 20 is heated. A vapour feed 32 is connected tothe heat exchanger 31 and opens out in the upper region, preferably thetop region, of a column 33. The column 33 has a multitude of packings 34which are usually configured as trays. In the lower region of the column33 is disposed the column bottom 35 from which a bottoms outlet 36 leadsinto the heat exchanger 31 and heats the streams conducted through theoutlet 223 into the heat exchanger 31. A pure product line 37 isconnected to the heat exchanger 31, which is followed downstream by theamidation 40. In the top region of the column 33 is disposed a topsoutlet 38 which opens into a heat exchanger 39 to which a vacuum pump310 is connected and opens in turn into a heat exchanger 311. Both theheat exchanger 39 and the heat exchanger 311 are connected via lines toa cooling vessel 312 to which a recycle line 313 is connected and isconnected to the loop reactor 26 in the acetone cyanohydrin preparation20.

The amidation 40 depicted in FIG. 4 first has an acetone cyanohydrinfeed 41 and a sulphuric acid feed 42 which open into a loop reactor 43.The acetone cyanohydrin feed 41 connected to the acetone cyanohydrinworkup 30 opens into the circuit of the loop reactor 43 downstream of apump 44 and upstream of a mixer 45. Upstream of this pump 44, thesulphuric acid feed 42 opens out. The mixer 45 is followed downstream bya heat exchanger 46 which in turn opens into a gas separator 47 fromwhich, firstly, a gas outlet 48 and a feed 49 to a further loop reactor410 exit. The further loop reactor 410 or a third has a comparableconstruction to the first loop reactor 43. From the further loop reactor410, a feed 411 enters a heat exchanger 412 which is followed by a gasseparator 413, from which, firstly, a gas outlet 414 and an amide line415 exit, the latter leading to the esterification/hydrolysis 50/MAAplant 50 a.

FIG. 5 shows the esterification 50, in which a solvent line 51 whichconducts water and organic solvent, and an amide line 52 connected tothe amidation 40 open into a tank 53 which is heatable by a tank heater54. In addition, an alcohol line 55 shown with a broken line opens intothe tank 53. The alcohol line 55 opens out both in the upper and in thelower region of the tank 53. The first tank 53 is connected to a furthertank 53′, which has a further tank heater 54′, via an ester vapour line56 indicated by a line of dashes and dots. This further tank 53′ too isconnected to the alcohol line 55 both from the bottom and from the top.The ester vapour line 56 is connected to the upper region of the tank53′ and opens into a bottom 57 of a column 58. In addition, a line fordilute sulphuric acid 59 is present in the upper region of the tank 53′.A tank unit 510 encircled in a dotted ellipse is formed from a heatabletank 53 and 54 with alcohol line 55 and ester vapour line 56. It ispossible for one, two or more of such tank units to follow inbattery-like succession, each of these tank units 510 being connectedvia the ester vapour line 56 to the bottom 57 of the column 58. From thebottom 57 of the column 58, a high boiler line 511 also leads to thetank 53, in order to feed water and organic solvent back to theesterification. In the upper region, preferably the top, of the column58, a first heat exchanger 512 followed by a further phase separator 513are connected via a suitable line. Both at the top of the column 58 andin the first heat exchanger 512, a first stabilizer feed 514 (stabilizerindicated with “S”) and a further stabilizer feed 515 may be provided inorder to feed an inhibitor or stabilizer which prevents undesiredpolymerization. Connected to the further phase separator 513 is ascrubber 516 in whose lower region a solvent line 517 exits and opensout in the solvent line 51 via a heat exchanger 521. From the upperregion of the scrubber 516, a crude ester line exits and opens into theester workup 60. The spent acid line 59 exiting from the upper region ofthe tank 53′ or of the tank of the last tank unit 510 opens into aflotation vessel 519 for removal of the solids and constituentsinsoluble in the spent acid. From the flotation vessel 519, a spent acidoutlet 520 enters the sulphuric acid plant 100, and a low boiler vapourline 522 which conducts the low-boiling constituents, for further workupand recycling, enters the esterification.

The ester workup shown in FIG. 6 is connected to the esterification 50via a crude ester line 61, the crude ester feed 61 opening into themiddle region of a vacuum distillation column 62. This column 62 hascolumn internals 63 and a bottom heater 64 arranged in the lower regionof the column 62. From the lower region of the column 62 whichconstitutes the bottom of this column, an ester outlet 65 exits, opensinto the ester fine purification 70 and hence feeds the crude esterfreed of low boilers to the fine purification. In the upper region ofthe column 62, usually in the top, a first heat exchanger 66 isconnected via an outlet, as are one further heat exchanger or aplurality of heat exchangers 67 which are followed by a phase separator69. In the phase separator 69, the stream 68 and the mixture stemmingfrom the heat exchanger 67 is divided into organic and aqueousconstituents, a recycle line 611 in the upper region being connected tothe phase separator 69 and opening out in the upper region of the column62. In the lower region of the separator, a water outlet 610 is presentand opens into the esterification 50 in order to feed the water removedback to the esterification. A reduced-pressure generator 613 isconnected to the heat exchangers 66 and 67 via a reduced-pressure line612.

In FIG. 7, the ester outlet 65 stemming from the ester workup 60 opensinto a distillation column 71. This comprises a plurality of columninternals 71 and, in the lower region of the distillation column 71, acolumn bottom heater 73. From the top region of the distillation column71, a pure ester vapour line 74 enters a first heat exchanger 75 whichis followed by one (or more) further heat exchangers 76 which areconnected to a reduced-pressure generator 717. The outlet of the furtherheat exchanger 76 has a line from which, firstly, an ester recycle line77 opens into the upper region, or into the top, of the distillationcolumn 71. The ester recycle line 77 has a stabilizer metering point 79which is disposed in the ester recycle line 77 upstream of a mixer 78.Secondly, from the line of the further heat exchanger 76, a pure esteroutlet 710 exits. An additional heat exchanger 711 and another heatexchanger 712 are connected to this in series connection. These arefollowed by a molecular sieve vessel 713 which has molecular sievepackings 714. Purified further by the molecular sieve, the ultrapureester is transferred through the ultrapure ester outlet connected to themolecular sieve vessel into the further processing plant 80.

FIG. 8 is a section from FIG. 2, so that reference is made to the figuredescription for FIG. 2. In addition: the reaction mixture is fed into acooling region 85 bordered by a reaction mixture outlet 88 through aninjecting element 81 configured as a nozzle. The cooling region 85 has avolume symbolized by the dotted area, which does not include the volumesof the multitude of cooling elements which are symbolized by the hatchedareas and which are flowed through with coolant via the coolant inlet214 and the coolant outlet 82. It is discernible from FIG. 8 that thevolume of the cooling region symbolized by the dotted area is greaterthan the volume of the cooling elements symbolized by the hatched area.The deflecting elements or flow resistances 213 configured as baffleplates may be provided either at a cooler wall 89 surrounding thecooling region 85 or at the cooling elements 83. The arrangement andselection of the injecting elements 81 and of the deflecting elements213 can achieve cooling flow directions 87 deviating from a main flowdirection 86 shown by the dotted arrow, and hence bring about goodmixing of the reaction mixture in the cooling region 85.

EXAMPLES

In a reaction arrangement corresponding to FIG. 8 with a loop reactor26, HCN and acetone were reacted at 10° C. in the presence ofdiethylamine as a catalyst. The reaction conditions are shown in thetable below.

Volume of cooling region 85 relative Residence time Exam- to outervolume of Reaction in the heat Conversion ple cooling elements 83conditions exchanger 212 of ACH 1. 1:1.7 No flow 0.2 h 75% resistances213 2. 1:1.7 Flow 0.3 81% resistances 213 3. 1:0.7 No flow 0.5 h 83%resistances 213 4. 1:0.7 Flow 0.6 87% resistances 213

REFERENCE NUMERAL LIST

-   1 Plant system-   2 Sulphuric acid line-   3 Further sulphuric acid line-   4 Spent sulphuric acid line—ester-   5 Spent sulphuric acid line—acid-   6 Methacrylic acid line-   20 Acetone cyanohydrin preparation-   30 Acetone cyanohydrin workup-   40 Amidation-   50 Esterification-   50 a Hydrolysis-   60 Prepurification-   70 Fine purification-   80 Further processing plant-   90 Finishing-   100 Sulphuric acid plant-   21 Acetone vessel-   22 Hydrocyanic acid vessel-   23 Scrubbing tower-   24 Cooling elements-   25 Offgas lines-   26 Loop reactor-   27 Acetone feed-   28 Hydrocyanic acid feed-   29 Pump-   210 Catalyst feed-   211 Mixer-   212 Heat exchanger-   213 Flow resistance-   214 Cooling lines-   215 Heat exchanger-   216 Collecting vessel-   217 Nozzle-   218 Cooling circuit-   219 Heat exchanger-   220 Outlet-   221 Stabilizing vessel-   222 Sulphuric acid feed-   223 Outlet-   31 Heat exchanger-   32 Vapour feed-   33 Column-   34 Packings-   35 Column bottom with heat exchanger-   36 Bottoms outlet-   37 Pure product line-   38 Tops outlet-   39 Heat exchanger-   310 Vacuum pump-   311 Heat exchanger-   312 Cooling vessel-   313 Recycle line-   41 Acetone cyanohydrin feed-   42 Sulphuric acid feed-   43 Loop reactor-   44 Pump-   45 Mixer-   46 Heat exchanger-   47 Gas separator-   48 Gas outlet-   49 Feed-   410 Further loop reactor-   411 Feed-   412 Heat exchanger-   413 Gas separator-   414 Gas outlet-   415 Amide line-   51 Solvent line-   52 Amide line-   53 First tank-   54 First tank heater-   53′ Further tank-   54′ Further tank heater-   55 Alcohol line-   56 Ester vapour line-   57 Column bottom-   58 Column-   59 Spent acid line-   510 Tank unit-   511 High boiler line-   512 Heat exchanger-   513 Phase separator-   514 Stabilizer feed-   515 Further stabilizer feed-   516 Extraction column-   517 Solvent line-   518 Crude ester line-   519 Flotation vessel-   520 Spent acid outlet-   521 Heat exchanger-   522 Low boiler vapour line-   61 Crude ester line-   62 Vacuum distillation column-   63 Column internals-   64 Bottom heater-   65 Ester outlet-   66 Heat exchanger-   67 Heat exchanger-   68 Water feed-   69 Phase separator-   610 Water outlet-   611 Recycle line-   612 Reduced-pressure line-   613 Reduced-pressure generator-   71 Distillation column-   72 Column internals-   73 Column bottom heater-   74 Pure ester vapour line-   75 First heat exchanger-   76 Further heat exchanger-   77 Ester recycle line-   78 Mixer-   79 Stabilizer metering point-   710 Pure ester outlet-   711 Additional heat exchanger-   712 Other heat exchanger-   713 Molecular sieve vessel-   714 Molecular sieve packings-   715 Ultrapure ester outlet-   716 High boiler line-   717 Reduced-pressure generator-   81 Injecting element-   82 Coolant outlet-   83 Cooling element-   84 Product outlet-   85 Cooling region-   86 Main flow direction-   87 Cooling flow direction-   88 Reaction mixture outlet-   89 Cooler wall

1. A process for preparing acetone cyanohydrin, comprising: A.contacting acetone and hydrocyanic acid in a reactor to give a reactionmixture, the reaction mixture being circulated, to obtain acetonecyanohydrin; B. cooling at least some of the reaction mixture by flowingit through a cooling region of a cooler, the cooler including onecooling element or at least two cooling elements; and C. discharging atleast a portion of the acetone cyanohydrin obtained from the reactor,wherein the volume of the cooling region of the cooler based on thetotal internal volume of the cooler is greater than the volume of thecooling element or of the at least two cooling elements of the cooler.2. The process according to claim 1, wherein at least some of thereaction mixture flows in a cooler flow direction different from themain flow direction at least during the cooling.
 3. The processaccording to claim 2, wherein the cooler flow direction is obtained bydeflecting the reaction mixture.
 4. The process according to claim 3,wherein the deflection is effected by a deflecting means provided in thecooler or connected to the cooler.
 5. The process according to claim 4,wherein the deflecting means is an injecting element or a baffle elementor both.
 6. The process according to claim 4, wherein the deflectingmeans is provided in the cooling region.
 7. The process according toclaim 1, wherein the cooling element is an elongated hollow body whichcan be flowed through by coolant.
 8. The process according to claim 7,wherein the cooling element has a rod-shaped or plate-shapedconfiguration.
 9. The process according to claim 7, wherein the coolingelement is a tube bundle.
 10. The process according to claim 1, whereinthe residence time of the reaction mixture in the cooler is in a rangeof about 720 to 5400 seconds.
 11. A process for preparing an alkylmethacrylate, comprising: a. preparing an acetone cyanohydrin by aprocess according to claim 1; b. contacting the acetone cyanohydrin withan inorganic acid to obtain a methacrylamide; c. contacting themethacrylamide with an alcohol to obtain an alkyl methacrylate; and d.optionally purifying the alkyl methacrylate.
 12. A process for preparingmethacrylic acid, comprising: α) preparing an acetone cyanohydrin by aprocess according to claim 1; β) contacting the acetone cyanohydrin withan inorganic acid to obtain a methacrylamide; and γ) reacting themethacrylamide with water to give methacrylic acid.
 13. An apparatus forpreparing alkyl methacrylates, comprising, connected to one another influid-conducting form: a plant element for preparing acetonecyanohydrin, followed by; a plant element for preparing methacrylamide,followed by; a plant element for preparing alkyl methacrylate,optionally followed by; a plant element for purifying the alkylmethacrylate, optionally followed by; a plant element forpolymerization, optionally followed by; and a plant part for finishing,wherein the plant element for preparing acetone cyanohydrin comprises aloop reactor with a cooler, and the cooler comprises a cooling regionwhich can be flowed through and a cooling element.
 14. The apparatusaccording to claim 13, wherein the cooler has a deflecting means. 15.The apparatus according to claim 14, wherein the deflecting means is aninjecting element or a baffle element.
 16. The apparatus according toclaim 13, wherein the deflecting means is provided in the coolingregion.
 17. The apparatus according to claim 13, wherein the coolingelement is an elongated hollow body which can be flowed through bycoolant.
 18. The apparatus according to claim 17, wherein the coolingelement has rod-shaped or plate-shaped cooling regions.
 19. Theapparatus according to claim 17, wherein the cooling element is a tubebundle.
 20. The apparatus according to claim 13, wherein the volume ofthe cooling region of the cooler based on the total volume of the cooleris greater than the volume of the cooling element or of the coolingelements of the cooler.
 21. A process for preparing an alkylmethacrylate, which is effected in an apparatus according to claim 13.22. A process for preparing polymers based at least partly on alkylmethacrylates, comprising the steps of: a) preparing an alkylmethacrylate by a process according to claim 1; b) polymerizing thealkyl methacrylate and optionally a comonomer; and c) working up thealkyl methacrylate.
 23. The process according to claim 22, wherein thepolymerization is effected by free-radical polymerization.
 24. Fibres,films, coatings, moulding compositions, mouldings, papermakingauxiliaries, leather auxiliaries, flocculants and drilling additiveswhich are based on an alkyl methacrylate obtainable by a processaccording to claim
 11. 25. Fibres, films, coatings, mouldingcompositions, mouldings, papermaking auxiliaries, leather auxiliaries,flocculants and drilling additives which are based on a methacrylic acidobtainable by a process according to claim 12.